When Republican Senator Rob Portman announced his support for gay marriage Friday, some critics said it shouldn't have taken his son's coming out as gay to make him stand up for LGBT rights. But Portman's not alone: in fact, knowing a gay person is one of the biggest factors that determines support for marriage equality — and its influence is growing. More and more, it looks like Harvey Milk was right when he said that those who opposed LGBT rights would change their minds as people close to them came out.

In 2003, 39% of people with a close friend, family member, or colleague who was gay supported gay marriage, according to Pew research. Just twenty-one percent who didn't know a gay person well supported it. Over the next four years, the gap widened — as of 2007, 55% of those who knew a gay person were in favor, compared to 25% of those who didn't. So Americans who know a gay person are increasingly coming to favor gay marriage, while support among those who don't know anyone is close to stagnant.

CORRECTION: The data illustrate the correlation between knowing a gay person and supporting gay marriage. An earlier title of the chart above overstated that relationship.